Halftoning is a technique commonly used to convert a rasterized image into an output image. Halftoning techniques address certain limitations of binary printing devices, which operate by turning a pixel “on” or “off” without the capability of depicting intermediate intensity or “gray” levels. Halftoning techniques make it possible for such devices to depict multiple gray levels by representing features in a printed image using patterns of small dots. If the human eye views a pattern of sufficiently small dots, the viewer generally does not perceive the individual dots but instead perceives the average gray level of the pattern. Halftoning techniques accordingly depict different regions within an image by using different patterns of small dots chosen to have an appropriate average gray level. A binary printing device typically generates patterns of dots by selecting an array of pixels, referred to as a halftone “screen”, and selectively turning the pixels within the screen on or off to produce a variety of patterns. A pattern generated in this way is referred to herein as a “halftone pattern”. Each halftone pattern corresponds to and is used to represent a particular gray level. The number of gray levels that can be rendered by a given halftone screen is limited by the number of pixels in the screen. On a binary printing device, an N-by-N halftone screen can depict N2+1 different gray levels. The average gray level represented by a particular halftone pattern is commonly referred to simply as the gray level of the halftone pattern.
The desired gray level is not the only factor considered in selecting a halftone pattern or a halftone screen. The halftone pattern or the size of the halftone screen may also be adjusted within an image to optimize image resolution, to minimize artifacts, or for other reasons. Where a printing system produces an image containing several regions representing different “modes”, i.e., photo, text and/or graphics, it may be preferable to vary the size of the halftone screen within the image based on the desired degree of spatial resolution in each region. The degree of spatial resolution provided by a halftone screen is determined by its “screen frequency”. Typically, a smaller halftone screen has a higher screen frequency. Subpixels generated by applying pulse-width modulation techniques can be used to create halftone screens as small as one pixel in size.
To render text or line art, it is often preferable to use a high-frequency screen having a high spatial resolution to avoid the appearance of artifacts such as jagged edges in the printed image; however, the use of a high-frequency screen tends to limit the number of gray levels that can be generated. Photographic images, in contrast, tend to contain many gray levels but require lower spatial resolution than is needed for text images. Consequently, a lower-frequency screen which permits the rendition of a greater number of gray levels, but offers lower spatial resolution, may be preferable for a region of an image containing a photographic image.
The use of different halftone patterns, or the use of multiple halftone screens of different sizes within a halftoned image can cause some printing devices to produce defects in regions of the printed image where more than one halftone screen or pattern are used. Some defects are printer-dependent and in many cases their appearance is unpredictable. Common techniques that apply different halftone patterns having the same screen frequency to render multiple gray levels generally produce a smooth transition in gray level from darker to lighter regions; however, in some printing devices a defect known as a “discontinuity” may appear in which an abrupt change from one gray level to another is visible within the transition region. Another technique uses halftone patterns generated using a high-frequency halftone screen to render a darker region but selects a lower-frequency halftone screen to render a lighter region. If the high-frequency and low-frequency halftone screens are carefully chosen, this technique produces a smooth transition from the darker region to the lighter region; however, in some printing devices a defect known as a “divergence” may appear in which the printing device fails to depict accurately the gray level of one or more halftone patterns in the transition region, and the shift from one screen frequency to another is visible.
An additional problem associated with some high-frequency halftone screens is instability. Printing devices that use electric charges in the printing process, including many toner-transfer printing devices, sometimes fail to print very small, isolated dots and therefore may fail to print one or more dots within a halftone pattern. A halftone screen that displays such behavior is referred to as being unstable. Instability represents an obstacle to achieving fine control over the gray level in a printed image, and can be especially problematic when a high-frequency halftone screen is used to depict lighter regions in an image. One solution commonly used is to select a relatively stable, lower-frequency halftone screen to render lighter regions; however, if such a selection necessitates a transition from a high-frequency to a low-frequency halftone screen, a divergence may appear within the printed image. There is a need to overcome the problems outlined above and to develop a technique permitting the use of different halftone screens and halftone patterns to achieve consistent and accurate rendition of multiple gray levels and of transitions between gray levels in a printed image while minimizing the occurrence of defects.